There has been a recent resurgence in the interest in unmanned aerial vehicles (UAVs) for performing a variety of missions where the use of manned flight vehicles is not deemed appropriate, for whatever reason. Such missions include surveillance, reconnaissance, target acquisition and/or designation, data acquisition, communications datalinking, decoy, jamming, harassment, or one-way supply flights. This interest has focused mainly on UAVs having the archetypical airplane configuration, i.e., fuselage, wings having horizontally mounted engines for translation flight, and empennage, as opposed to "rotor-type" UAVs, for several reasons.
First, the design, fabrication, and operation of "winged" UAVs is but an extrapolation of the manned flight vehicle art, and therefore, may be accomplished in a relatively straightforward and cost effective manner. In particular, the aerodynamic characteristics of such UAVs are well documented such that the pilotage (flight operation) of such vehicles, whether by remote communications datalinking to and/or software programming of an on-board flight computer, is relatively simple.
Additionally, the range and speed of such UAVs is generally superior to rotor-type UAVs. Moreover, the weight-carrying capacity of such UAVs is generally greater than rotor-type UAVs such that winged UAVs may carry a larger mission payload and/or a larger fuel supply. These characteristics make winged UAVs more suitable than rotor-type UAVs for certain mission profiles involving endurance and distance. Winged UAVs, however, have one glaring deficiency that severely limits their utility.
More specifically, winged UAVs do not have a fixed spatial point "loiter" capability. For optimal performance of many of the mission profiles described hereinabove, it is desirable that the UAV have the capability to maintain a fixed spatial frame of reference with respect to static ground points for extended periods of time, e.g., target designation. One skilled in the art will appreciate that the flight characteristics of winged UAVs are such that winged UAVs cannot maintain a fixed spatial frame of reference with respect to static ground points, i.e., loiter. Therefore, mission equipment of winged UAVs must utilize complex and costly motion-compensating means to suitably perform such mission profiles.
Rotor-type UAVs, in contrast, are aerodynamically suited for such loiter-type mission profiles. The rotor subsystems of such UAVs may be operated so that the vehicle hovers at a fixed spatial frame of reference with respect to a static ground point. Prior art ducted rotor-type UAV designs, however, were generally deficient in that such UAVs experienced nose-up pitching moments in translational flight. Several examples of such vehicles are described in commonly-owned, co-pending application Ser. No. 07/526,092, filed May 18, 1990, entitled AN UNMANNED FLIGHT VEHICLE INCLUDING COUNTER ROTATING ROTORS POSITIONED WITHIN A TOROIDAL SHROUD AND OPERABLE TO PROVIDE ALL REQUIRED VEHICLE FLIGHT CONTROLS. In general, many prior art ducted rotor-type UAVs incorporated ancillary aerodynamic structures, with or without control surfaces, into the UAVs to counteract such nose-up pitching moments. These vehicles are, in essence, hybrids of winged and rotor-type UAVs.
While the utilization of such ancillary aerodynamic structures represents one possible solution to the nose-up pitching problem, such a solution impairs the overall performance of the UAVs. Aerodynamically, such ancillary aerodynamic structures increase the overall drag characteristics of the vehicle. In addition, such ancillary aerodynamic structures add to the gross vehicle structural weight. Both of these factors may necessitate the utilization of a higher horsepower powerplant (or a reduction in vehicle flight range and/or speed).
Another possible solution is the utilization of cyclic pitch to counteract the nose-up pitching moment experienced by ducted UAVs in forward translational flight. The utilization of cyclic pitch to counteract the nose-up pitching moments is described in in commonly-owned, co-pending application Ser. No. 07/526,092, filed May 18, 1990, entitled AN UNMANNED FLIGHT VEHICLE INCLUDING COUNTER ROTATING ROTORS POSITIONED WITHIN A TOROIDAL SHROUD AND OPERABLE TO PROVIDE ALL REQUIRED VEHICLE FLIGHT CONTROLS. While the utilization of cyclic pitch to counteract the fuselage-induced nose-up pitching moment experience by ducted UAVs in forward translational flight is feasible, a performance penalty is incurred in the form of lost lift which requires an increase in engine power output to augment the lift via the rotor assembly. Further details regarding the utilization of cyclic pitch in this manner are set forth hereinbelow.
A need exists for a means of counteracting the undesirable nose-up pitching moments experienced by ducted rotary-type UAVs in forward translational flight. Such means should minimize cyclic trim pitch requirements and rotor assembly power requirements while concomitantly providing high hover efficiency.